1. Field of the Invention
The present invention relates to a camera having a light blocking device.
2. Description of the Related Art
It has heretofore been proposed to provide a camera which is arranged to divide a light flux exiting from a photographic lens into two light fluxes by means of a half-mirror and conduct the two light fluxes in different directions, one being conducted in a direction in which to expose film and to detect focus of the photographic lens and the other toward a viewfinder or an image pickup element.
FIG. 4 is a diagrammatic cross-sectional view showing the central portion of a single-lens reflex camera having a fixed half-mirror, which camera is disclosed in Japanese Laid-Open Patent Application No. Hei 2-89039. In the shown central portion, a light flux indicative of an image of a subject exiting from a photographic lens (not shown) is divided into two light fluxes to be respectively conducted toward a viewfinder system and a film plane, by means of a semi-transparent fixed mirror 1. The fixed mirror 1 is mounted on a mirror frame 2 by adhesion.
A sub-mirror 7 is provided for conducting the light flux transmitted through the fixed mirror 1, toward a focus detecting device 30. The sub-mirror 7 is mounted on a sub-mirror mount 8 by adhesion, and the sub-mirror mount 8 is supported for rotation about a shaft 9 by a mirror driving plate 10.
When the camera is in its photography standby state, the sub-mirror mount 8 is urged clockwise by a spring 14 and is held at a predetermined position by a stopper 15, as shown in FIG. 4.
When a photographic (exposure) cycle is started, the mirror driving plate 10 starts to rotate clockwise about a shaft 11 and the sub-mirror mount 8 also starts to move down toward the bottom of a mirror box. When approaching the bottom of the mirror box, the sub-mirror mount 8 is urged counterclockwise by a spring 17. The sub-mirror mount 8 continues to move down against the counterclockwise urging force of the spring 17, and when the sub-mirror mount 8 reaches its lower position, the sub-mirror 7 is placed into a retracted state in which it is held approximately in parallel with the bottom of the mirror box.
A sub-mirror driving mechanism using a conventional fixed half-mirror is constructed as described above in brief.
In the above-described conventional construction, a portion of the light flux transmitted through the semi-transparent fixed mirror 1 is blocked by the sub-mirror 7, while the other portion of the light flux reaches a shutter curtain 18. For this reason, the light blocking performance of the shutter curtain 18 needs to be higher than those of conventional general single-lens reflex cameras. To increase the light blocking performance of the shutter curtain 18, it is customary to adopt a double light blocking system in which both leading and trailing curtains of a shutter are used to block light.
However, to achieve such double light blocking, it is necessary to prepare an exclusive shutter having a complicated mechanism and an exclusive control program for controlling such exclusive shutter. As a result, the operation of the shutter is complex so that the probability of occurrence of faults increases, and the shutter or the camera becomes expensive.
If a general focal plane shutter is employed, it will be impossible to fully block the incidence of light from the photographic lens on the film plane. As a result, stray light may enter the film plane to adversely affect photography.